Rangewide Climate Vulnerability Assessment for Threatened Bull Trout

Across the western United States, cold-water streams wind their way through the landscape. These gin clear waters are home to the Bull Trout, a little-known cousin of Pacific salmon native to the Pacific Northwest. The Bull Trout was not described scientifically until 1978, but is now recognized as a threatened species. Historically the species was harvested heavily in some locations, but today anglers practice catch and release in attempts to preserve declining populations. Perhaps more importantly, Bull Trout depend on the most important resource for all of us in the Pacific Northwest – dependable availability of cold, clean water. Bull Trout are the most cold-water dependent fish in the Pacific Northwest and are extremely sensitive to winter floods that flush incubating eggs and fry out of gravel nests. Climate warming will spell trouble for Bull Trout if it leads to warming of streams and increased flooding during the winter.

Dunham and his colleagues, David Hockman-Wert, Nathan Chelgren, and Michael Heck, from the USGS, Dan Isaak from the US Forest Service, and Seth Wenger from the University of Georgia have addressed how these climate-related threats influence Bull Trout across the southern extent of the species’ range, including Oregon, Washington, Idaho, Nevada, and Montana. Using stream temperature predictions across the southern portion of the range, researchers mapped out cold-water streams or “patches” on the landscape that meet the suitability requirements of the species.

“This is the first opportunity we’ve had to take an objective look at Bull Trout across the species’ range and, although we do have critical habitat designated and temperature models available, we’ve never precisely defined what that landscape looks like for Bull Trout across the whole species range,” said lead scientist, Jason Dunham. “To me, that is the fundamental game board that we need to know in terms of how we are going to work with the species in the long-term.”

This patch based mapping approach adopted by Dunham and his colleagues gives researchers and natural resource managers an extremely detailed picture of the interconnected networks of streams suitable to support Bull Trout spawning and early rearing. “The patch based view of the landscape is totally different than what we would normally use. Normally we would rely on hydrologic units.” Said Dunham. Hydrologic Unit Codes, more commonly referred to as HUCs, are used to identify the geographic area and boundaries of watersheds and are designed to be homogeneous pieces of the landscape. “In contrast, patches show tremendous variability in their size, connectivity, and physically where they are located on the landscape. Those three characteristics fundamentally drive where you see Bull Trout – you lose all that information when you just look at HUCs.” Said Dunham.

In addition to locating and mapping patches suitable to support Bull Trout, Dunham and his team analyzed a suite of climatic and local factors hypothesized to influence Bull Trout presence in the region. Of the twelve variables evaluated, four were most important for determining Bull Trout presence: larger patches of cold water, the presence of extremely cold water (<10°C) temperatures in summer, fewer floods in winter, and low levels of human disturbance. Three of these variables (the size of stream networks, cold water presence, and winter flooding) are strongly sensitive to changing climates, highlighting the vulnerability of Bull Trout to climate change.

The results of Dunham’s research enable us to better understand where different threats are working to influence Bull Trout and help us identify effective conservation and management strategies to ensure the species can persist and thrive in the face of climate change.

“At a higher level this research allows regulatory and land management agencies like the US Fish and Wildlife Service, US Forest Service, Bureau of Land Management, and the National Park Service to take a look at how their landscapes fit into the bigger picture for the species as a whole.” Said Dunham.

While the big picture view is extremely useful, it is not detailed enough to make conservation decisions at the local level. Dunham and his team are not always aware of small barriers within each watershed that prevent or hinder Bull Trout distribution. There may be culverts that fish cannot swim through, or localized regions where invasive species are especially problematic. “We don’t yet have the capability to map those barriers at high-resolution and at a broad extent. This is a limitation in terms of how deep we can drill across the range in a unified way.” Said Dunham.

That’s where the local managers step in. Their knowledge of localized barriers within core areas and watersheds can be used alongside the big picture view that Dunham and his team have provided to implement Bull Trout management and restoration efforts at the local scale.

Their hope is that their tool will help inform conservation and management efforts across the southern extent of the Bull Trout range, and ensure that this elusive predator will continue to travel the expansive river network system of the Pacific Northwest for years to come.

The bull trout is the most cold-adapted fish in freshwaters of the Pacific Northwest. The species is listed as threatened under the U.S. Endangered Species Act, but climate warming may place the species at further risk. Climate warming may lead to warming of streams in summer and increasing probability of floods in winter, leading to widespread loss of habitat projected for bull trout. This project sought to further elaborate how these climate-related threats influence bull trout across five western states (OR, WA, ID, MT, NV). These states form the southern margin of the species’ range. We used predictions of temperatures in streams across approximately two-thirds of this extent to map coldwater streams or “patches” suitable for spawning and early rearing of bull trout. Our results indicate that larger patches of cold water were much more likely to support the species. We also found that bull trout were more likely present in patches with extremely cold (<10C or 50F) temperatures in summer (August), fewer floods in winter, and low human influences as measured by the Human Footprint index. In addition to elucidating the importance of local and climate-related threats, our work has identified dozens of places where bull trout may exist, but have not yet been detected, as well as other places where bull trout have been observed recently, but may be at high risk of local extinction. Future work will focus on completing these analyses across the range of bull trout in the conterminous United States.

Recovery planning using the Bull Trout Vulnerability Assessment: This project created a probability model of bull trout presence across its entire range and mapped suitable habitat patches using data on stream temperature, known fish presence, local threats, habitat connectivity, and climate sensitivity. This model is being used in the official recovery plan for the species.

Call to collect additional data: This project identified knowledge gaps, from habitat connectivity data to population surveys, that need to be filled to make informed management decisions on bull trout recovery programs.